chore: add typecheck script and fix pre-existing TypeScript errors

- Add `typecheck` script (`tsc --noEmit`) to package.json
- Remove conflicting `declare global` from test/setup.ts (superseded
  by @cloudflare/workers-types); use `globalThis as any` for test globals
  and declare minimal `require` locally to avoid pulling in @types/node
- Cast `createMockEnv()` and `deleteRes.json()` results in admin.test.ts
  to silence strict `unknown` / MockKV-vs-KVNamespace errors

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -10,7 +10,8 @@
     "deploy": "wrangler deploy --env production",
     "test": "vitest run",
     "test:watch": "vitest",
-    "test:coverage": "vitest run --coverage"
+    "test:coverage": "vitest run --coverage",
+    "typecheck": "tsc --noEmit"
   },
   "author": "",
   "license": "MIT",